In papermaking, it is important that the hardness of the manufactured paper roll in the axial direction be maintained at a uniform level, since that is an indication of the uniformity of the overall manufacturing process. The reeled paper rolls on modern papermaking equipment can be on the order of 100 inches or more in diameter and from 200 to 300 inches or more in length. Even rolls which are slit and rewound for printing can be on the order of 40 inches or more in diameter and 36 to 72 inches in length. In order to gauge the uniformity of the manufacturing process, and also as a predictor of paper performance in printing, it is useful to test these paper rolls for hardness and hardness uniformity across the length of the roll.
A hand-held device which has long been used for such testing is described in Pfeiffer U.S. Pat. No. 3,425,267, and that device will sometimes be referred to herein as a Rho Meter. It utilizes the principal of measuring the peak deceleration value of a striker having a known mass, which is projected radially against the wound roll of paper from a fixed height under the impetus of a spring having a known spring constant. Thus, the '267 reference provides a reading for peak deceleration of a striker which impacts the wound paper roll with a known velocity. The hardness value is expressed in Rho units (derived from the Greek letter .rho. which is used to represent density), and is an arbitrary scale which has received rather wide acceptance in the papermaking industry. While the instrument does give repeatable results if used properly, it also presents a number of difficulties, particularly in high volume use.
It is extremely important in using the Rho Meter described in the '267 patent to assure that the instrument is perfectly aligned with the roll of paper to be tested, such that the runners on which the device contacts the paper are exactly tangent to the paper roll at the centerline of the striker. If the runners are not tangent to the roll, or if the tangent is located otherwise than at the striker centerline, the force of the striker will not be perpendicular with respect to the paper roll, and the expected velocity of impact will not be achieved, resulting in erroneous readings.
A further difficulty with the '267 device is the fact that a spring-type mechanism controls the impact of the striker against the roll, and the force exerted by the mechanism is, in part, related to the speed at which the trigger of the device is actuated. Thus, a user must pull the trigger slowly and at a consistent speed in order to get reproducible results from reading-to-reading and day-to-day.
Because the system utilizes a spring driving a mass, it is orientation sensitive. In the preferred mode, the device must be used with the striker motion precisely vertical, to eliminate velocity variations due to gravity effects on the weight. Thus, the ability to use the device consistently and according to its intended operating mode presupposes that the operator has access to the very top of the paper roll. That may not be difficult when testing 40-inch rolls, but when the rolls of paper are six, eight or more feet in diameter, taking of readings in the expected instrument orientation can become a problem.
Finally, while the '267 device is configured to latch the reading from the last test on an analog meter which is an integral part of the instrument. A commercial version of that instrument has also been configured with an analog jack for driving a strip chart recorder or the like, so that the operator can take a number of readings and produce a strip chart type roll profile without the need for manually recording the Rho numbers. While that does present a measure of convenience, the operator must still exercise care in the manipulative steps discussed above, in order to assure a constant force hit. The strip chart record is also substantially less useful than a complete set of readings which can be graphed or otherwise statistically analyzed.
A roll hardness testing device which does not suffer from many of the manipulative limitations of the '267 device is the simple backtender's stick. That device is a hardwood stick which the backtender's stick (the operator at the reel end of the papermaking machine) uses to strike the roll of paper and gauge (usually by both ear and touch) the frequency and amplitude of the resulting vibrations. The operator attempts to strike the roll with a predetermined force, and the sound produced by the backtender's stick hitting the roll (as well as the vibrations in the roll felt by the hand of the operator) are interpreted subjectively by the operator as a measure of roll tightness. That procedure is obviously very prone to inaccuracies introduced from operator-to-operator or even by a given operator over the course of a day. The technique is not known to be quantitative in any reasonable sense of the word.